The overall objective is to elucidate the biochemical and subcellular mechanism of action ryanodine, a drug which depresses contractile force in heart. Since ryanodine apparently alters intracellular Ca 2 ions metabolism, we intend to study the effects of ryanodine on basic cellular processes which regulate ionized Ca 2 ion concentration in cardiac tissues. Our preliminary data suggest that the specific site of ryanodine action in heart may be sarcoplasmic reticulum, where the drug acts to increase Ca 2 ion storage. Effects of ryanodine on heart will be conducted at several levels of tissue organization ranging from whole tissues studies (effects of ryanodine on contractility, electrophysiology, and 45Ca 2 ion exchange) to studies on purified membrane preparations of sarcoplasmic reticulum and sarcolemma (effects of ryanodine on ionic fluxes and Ca 2 ion-handing, enzymic activities, and generation of transmembrane potentials). Detailed species and tissue comparative studies on effects on contractility correlated with effects on properties of the membranes in vitro should allow definition of the mechanism of drug action. If ryanodine is shown to depress contracility by blocking Ca 2 ion efflux channels in sarcoplasmic reticulum, the usefulness of this drug as a powerful and specific molecular probe will be established. As a result of these studies, knowledge will be gained on basic mechanisms of excitation-contraction coupling in heart.